The present invention relates generally to refractometry and, more particularly, concerns a method and apparatus for improving the accuracy and reliability of critical-angle refractometry measurements.
The purpose of a refractometer is to measure the refractive index of a sample material M, for example, a liquid. A prior art critical-angle refractometer is shown schematically in FIG. 1. The liquid sample M is typically contained in a dish in which one surface of an optical prism P, having a high index of refraction, forms the floor of the dish. Rays of light R from a Light Source S are focused by a Lens L1 into prism P. It is characteristic of the interface between two disparate optical media that a light beam entering the interface will be, in part, refracted or bent upon traversing the interface, and it will be, in part, reflected from the interface. However, for any interface, there is a “critical-angle” such that a light beam incident on the interface with that or a greater angle to the normal will be totally reflected. The critical angle is defined by Equation (1).sin(critical angle)=(refractive index of sample)/(refractive index of prism)  (1)
By design of the prism/sample interface I in FIG. 1, rays R incident on interface I are at least partially reflected therefrom. The reflected rays exit the prism and are directed by a lens L2 onto a linear optical detector O. The detector O is a linear array of light sensitive pixels, each of which produces an electrical signal proportional to the level of light incident on it. Each pixel receives a bundle of light rays that reflect from interface I at a particular angle of incidence. If this angle of incidence is greater than the critical angle, then 100% of the rays in that bundle are reflected from the interface I. This is called total internal reflection (TIR). If this angle of incidence is less than the critical angle, then the rays are only partially reflected by the prism/sample interface, because a portion of the light is refracted into the liquid sample and, ideally, does not reach the detector. A pixel receiving partially reflected light will sense a lower intensity of light than a pixel receiving totally reflected light. The light intensity pattern on the array of pixels therefore provides a map of the amount of reflection at the prism/sample interface of the bundle of light beams corresponding to each pixel.
From the light intensity detected at each pixel (and the associated angle of incidence of the light beams it receives), the reflectance at the prism/sample interface I can be calculated and the result represented as a graph of reflectance versus angle of incidence (or pixel), i.e. a reflectance graph. An exemplary reflectance graph is illustrated in FIG. 2. As can be seen, the graph exhibits 100% reflection (TIR) over a range of pixels corresponding to low angles of incidence relative to interface I (high angles of incidence relative to the normal) and then transitions sharply to a relatively low percentage value of reflectance. The determination of the refractive index of a sample is performed by locating the pixel where the transition from total to partial reflection occurs in the graph of FIG. 2 and using the associated angle of incidence, together with the critical angle equation (1) above, to solve for refractive index. A convenient way to locate the transition is to search for the region of pixels where the reflectance has the greatest slope or rate of change. Because of the limited pixel resolution of existing detectors, interpolation between the pixels is usually also required. The range of refractive index values that can be measured is a function of the range of angles of incidence of the light rays that reach the detector, and a function of the refractive index of the prism material. A typical range that can be measured is refractive indices from 1.3 to 1.7.
Those skilled in the art will appreciate that, although the reflectance graph is shown visually in FIG. 2 for illustrative purposes, it need not exist in that form. For example, it may merely be a collection of data in a computer which can be processed in the manner described herein.
With existing refractometers, a number of operating conditions may lead to a false result being reported. For example, the user may not have cleaned the prism adequately prior to loading the liquid sample. This can result in a mixture of materials at the prism/sample interface. Sometimes, the user simply forgets to load a new sample and, instead, accidentally performs a duplicate measurement on the previously loaded sample. Another difficulty arises when an insufficient quantity of liquid sample is placed in the prism dish. If the quantity of liquid in the dish is too small, the upper free surface (or meniscus) of the liquid sample may be close enough to interface I to reflect some of the light back into the system that ideally should not reach the detector. This will result in incorrect reflectances being calculated for the pixels receiving this additional light. Users may also attempt to measure samples that cannot be well characterized by a single refractive index, for example inhomogeneous materials like colloidal suspensions. Finally, users may attempt to measure samples that have refractive indices that are outside the range of refractive index that the instrument can measure.
Broadly, it is an object of the present invention to avoid shortcomings of existing refractometry and methods and systems. It is specifically contemplated that the invention should prevent users from making false measurements in all of the situations discussed above.
In accordance with one aspect of the present invention, a critical-angle refractometer which utilizes an in image of light reflected from an optical interface with a vessel containing a sample under test to determine an optical property of the sample, sample properties are evaluated to prevent improper testing of the sample. This evaluation includes establishing reflectance information associating the amount of reflection with locations in the image; and utilizing a plurality of properties of the reflectance information to determine if the vessel contains a proper sample under test.
Preferably, the properties include the point of transition from complete to partial reflection and at least one of the following: the average rate of change of reflectance as a function of position in the image in a predefined range of interest (ROI) about the point of transition; the maximum reflectance in the ROI; the minimum reflectance in the ROI; the range of reflectance in the ROI; and the average reflectance in the ROI.